The present invention relates to an apparatus for protecting equipment connected to a high voltage direct current line, said apparatus being configured to obtain said protection by having ground connection means enabling connecting of said line to the ground, said ground connecting means comprising a current valve having at least one semiconductor device of turn-off type connected with a forward conducting direction thereof towards ground and at least one rectifying member connected in anti-parallel therewith, said apparatus further comprising a control unit configured to control said at least one semiconductor device to conduct for draining current from said line and by that protecting a said equipment.
“High voltage direct current line” is to be interpreted broadly and includes in this context any line feeding electric power in the form of a direct current to any type of equipment and which is on high voltage potential, i.e. a voltage of at least 1 kV but normally higher, such as well in the order of 100 kV to 1200 kV. Thus, said line does not have to be a direct voltage network in a plant for transmitting electric power between converter stations, although that is a particular application of the present invention.
“Protecting” is here also to be interpreted broadly, and this may for example involve a prevention that the voltage on said line will reach too high levels or that the current fed to said equipment will exceed a predetermined level. Thus, it may be a question of obtaining a better or more preferred operation of said equipment by said “protection”, but it may also be a protection of the equipment for preventing failure thereof.
For illuminating the invention but not in any way restricting the scope thereof the case of such an apparatus connected to a direct voltage network interconnecting two converter stations of a plant for transmitting electric power in the form of High Voltage Direct Current will now be briefly discussed. The converter stations are each connected to an alternating voltage network, which may include generators of electric power and/or consumers of electric power, and the converter stations have converters adapted to convert direct voltage into alternating voltage and conversely. The level of the voltage of the direct voltage network is defined by one or more capacitors, and if this level gets too high said at least one semiconductor device of said current valve of the apparatus may be turned on and the capacitor may be discharged by draining current to the ground and by that the voltage on the direct voltage network may be reduced to a suitable level, whereupon the semiconductor device is turned off.
It may also happen that for instance a wind power plant connected to one converter station feeds a high amount of electric power, due to extreme conditions, through a first of the converter stations into the direct voltage network, but any consumer or other equipment connected to the other converter station may not be able to take care of this amount of power. Said at least one semiconductor device of the current valve of the apparatus may then be turned on for conducting current into the ground, so that said equipment will not be influenced by said extreme conditions of power generation.
In known apparatuses of the type defined in the introduction a linear resistor is connected in series with said current valve. It is of course a desire to improve such an apparatus already known by making it more reliable and/or less costly.